The Pigtronix Keymaster is an interface to connect any audio source (instrument, line level, mic) with pedals and return the signal at the correct impedance.
Basically, what you get is a pair of true bypass effects loops, each with its own footswitch, that can be run in series or parallel. In parallel mode, there's the option of crossfading between the two by adding an expression pedal.
On a pedalboard, the most obvious use is to bring in two different groups of pedals at any time.
But you can also get creative by running two different dirt pedals in parallel for an interesting blend, or with one loop muted, utilising a cross-fading expression pedal as a blender to turn up delay, for example.
For recording, the Keymaster is great for re-amping - running the recorded signal out to your pedals and back, or to an amp.